Which port number are you referring to, because JBoss services use a bunch of different ports.
Are you looking for
http - 8080
https - 8443
ajp13 - 8009
EJBInvoker/JRMPInvoker - 4443
JMS Invocation Layers - there are three different types, Across JVMs, in JVMs and http JMS invocation layer
NamingService - 1099
Non MulticastIP Clustering Naming Service 1100
MulticastIP Clustered Naming Service 1102
All use different ports
So which one, and also look into removing services or closing ports for services that you don't use.
If you really like to know about port address for the http as well as you want to change it to your specific way, for the jboss 4.x
i will suggest you to go to the below location.
%JBOSS_HOME%\server\default\deploy\jboss-web.deployer and open server.xml
else if you are using jboss 5.x you can go to the
%JBOSS_HOME%\server\default\deploy\jbossweb.sar and open server.xml. And after opening the file
try to find out the similar code written in the code section.
port="8080" can be changed with the user defined number but make sure you are not trying to use some reserved port and the other default ports which are mentioned and already allocated for the diffrent applications.